Stock Plans (Restricted Shares Outstanding) (Details) - Restricted Stock [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance at beginning of period (in shares)	633	549	462
Shares granted (in shares)	2,023	454	396
Shares earned (in shares)	(221)	(240)	(248)
Shares forfeited (in shares)	(577)	(130)	(61)
Balance at end of period (in shares)	1,858	633	549
Balance at beginning of period (in dollars per shares)	$ 58.63	$ 78.00	$ 88.95
Restricted stock granted (in dollars per shares)	8.26	47.77	65.40
Restricted stock vested (in dollars per shares)	66.82	80.86	78.90
Restricted stock forfeited (in dollars per shares)	16.47	60.92	76.65
Balance at end of period (in dollars per shares)	$ 16.02	$ 58.63	$ 78.00
Balance at beginning of period	$ 4	$ 28	$ 33
Restricted stock granted	5	3	20
Restricted stock vested	1	2	13
Balance at end of period	$ 4	$ 4	$ 28